Activated carbons prepared from agricultural waste were modified to introduce surface acid sites by treatment with HNO₃ or H₂SO₄. These acid-functionalised materials were studied as heterogeneous catalysts for the solvent-free acetalisation of acetone with glycerol to produce 2,2-dimethyl-1,3-dioxolane-4-methanol (solketal). The best catalyst was prepared by treatment with concentrated sulphuric acid and achieved 97% conversion of glycerol with high selectivity towards solketal by reaction with acetone at room temperature. The catalytic behaviour of the activated carbons was correlated with their physicochemical properties through characterization by N₂-physisorption, XPS, elemental analysis, TGA and Boehm titration of the acid sites.